The present invention relates to a method of forming paucilamellar lipid vesicles which have amorphous central cavities substantially filled with a water immiscible material such as an oil. The invention is a "cold-loading" technique which allows incorporation of volatile and/or heat labile (heat degraded) materials which could not otherwise be incorporated into the vesicles.
U.S. Pat. No. 4,911,928 describes a "hot-loading" method of making paucilamellar lipid vesicles with water immiscible material substantially filling the amorphous central cavities. The lipid (and any oil or water immiscible material to be incorporated) is heated to an elevated temperature, e.g., a liquid or flowable form, so that it can be injected into an excess of an aqueous phase. This injection of the lipid into the aqueous phase causes the formation of small lipid micelles (probably spheroidal) which aggregate upon cooling with turbulent or shear mixing. The aggregated micelles fuse into vesicles with multiple bilayer shells surrounding a central, amorphous core. If an oil or a water immiscible material is also present, both lipid micelles and microemulsion oil droplets are formed. The microemulsion oil droplets act as nuclei about which the micelles aggregate, forming an oil-filled amorphous central cavity of the vesicle surrounded by the lipid bilayers. Preferably, a small amount of an indifferent surfactant is also included to stabilize the oil. The term "indifferent surfactant," as used herein, means a surfactant which will not form lipid vesicles but is able to emulsify the water immiscible materials to be encapsulated. Indifferent surfactants include most polyoxyethylene sorbitan ethers (Tweens), sodium dodecyl sulphate, and C.sub.12 -C.sub.18 fatty acids and their salts such as sodium oleate. If an indifferent surfactant is not used, a portion of the wall-forming lipid is cannibalized to stabilize the oil.
Although the "hot-loading" method is effective for a large number of water immiscible materials, the method is not useful for a variety of important water immiscible materials which are too volatile or heat labile at the vesicle forming temperatures. If the "hot-loading" methods are tried for these thermolabile materials, the majority of the water immiscible material is volatilized, leaving only a small portion to be incorporated into the vesicle. These volatile materials include insecticides such as diethyltoluamide (DEET), certain perfumes and fragrances, flavor oils, as well as many other materials such as mineral spirits. Since some fragrances are mixtures, release of one part of the mixture can change the overall properties dramatically. Further, even certain non-volatiles are more easily introduced into the amorphous central cavities of vesicles using the present "cold-loading" technique than the "hot-loading" technique. For example, the cleaning agent d-limonene can be incorporated into vesicles at a relatively low concentration using "hot-loading" but a much higher concentration can be achieved using the "cold-loading" technique.
Accordingly, an object of the invention is to provide a method of "cold-loading" the amorphous central cavities of paucilamellar lipid vesicles with water immiscible materials.
Another objection of the invention is to provide a means of incorporating volatiles into paucilamellar lipid vesicles.
A further object of the invention is to provide a generalized means of loading lipid vesicles with oily or water immiscible material which can be used with phospholipid, ionic, and nonionic lipid materials.
Further objects and features of the invention will be apparent from the description and the Drawing.